Some formations are so soft that the weight of the drill string will push the drill head along a generally vertical axis without drill bit rotation. It is usually preferred to use a drill bit, with rock cutting structure, as a drill head. Drill bits usually have a plurality of jets that project, with slight divergence, along the axis of the drill string. To influence lateral control of the progressing well bore, one jet may be larger than other jets on the bit. When the drill string is not rotated, the large jet softens the formation more on the related side of the bit and the bit drifts toward the softer side. The large jet is usually oriented relative to a selected reference until some formation is displaced or softened, then the drill bit is rotated to follow through the direction influenced by the large jet. When the lateral influence has served the purpose, further lateral influence of the large jet can be nullified by rotating the drill string.
The use of jets to laterally influence the course of an advancing drill bit has included jets that project laterally to push the drill bit laterally. That arrangement, seldom used now, permitted optimum drilling jet arrangements in the drill bit and offers some advantage when harder formations are encountered. The drilling fluid lost through the lateral jet, however, reduces the possible benefit the extra fluid could provide if directed through the regular bit jets.
Considerable research and experience has evolved the optimum sizing and placement of the jets in the drill bit. Considerable penetration rate can be lost by compromising the optimum bit jet arrangement, when harder formations are encountered while drilling. If optimum bit jet arrangements are used in soft formations, the drill string my not have to be tripped to change the jets when harder formations are encountered. There is considerable advantage in providing an alternate way to urge the drill string to proceed along a laterally changing center line.
Some definitions are in order. Oil field parlance currently defines making well bore with jets, and little or no drill bit influence, as “jetting ahead”. Making well bore with a rotating drill bit is referred to as “drilling ahead”. In any case, the circulation of drilling fluid to remove cuttings, or debris, is taken for granted.
A drill head is the lower terminal of a drill string. The drill head is usually a drill bit of some form but, for use in very soft formations, it may be the equivalent of a bull plug. The drill head will usually have some form of nozzle to project a jet of drilling fluid.
Drilling fluid circulation is essential to well bore debris removal, and jet nozzles usually project a drilling fluid stream with characteristics adapted to the situation to minimize re-grinding of cuttings by the drill head. Such jets can be defined as circulation jets. Nozzle arrangements adapted to influence the course of a progressing well bore usually sacrifice some well bore cleaning ability and may be called lateral influence nozzles.
Several terms are used in the oil field to define a well bore axis at a particular location. The term “angle” is usually the angular difference between the earth vertical and the axis of the well bore. The term “direction” can be expressed in the aviators form (0 to 360 deg) or the maritime form in defined degrees from a prime earth compass direction. The term “course” related to the well bore indicates that both angle and direction is considered, but may not be quantified. The term “course” may be used without qualifiers when only change in the course is the primary consideration.